The deep freeze that has New Yorkers shivering in their boots has been a crime-fighting blessing for the NYPD — and as of midnight had given the city a nine-day streak without a murder, law-enforcement sources told The Post.

“Jack Frost is the policeman’s best friend,” said a source. “The criminals are staying indoors.”

The Big Apple’s last murder occurred Jan. 16 in The Bronx, where Jennifer Rivera, 20, and her uncle, Jason Rivera, 30, were gunned down execution-style while sitting a parked car, police said.

Rivera had a criminal drug history, sources said. No one has been arrested in connection with the slayings.

The murder-free streak ties the aftermath of Hurricane Sandy last year as the longest in recent memory, said NYPD officials.

“I’m sure the weather helps cut down on the numbers,” said a police source. “A lot of murders occur on the street. It’s too cold to hang out outside.”

But the source insisted homicides would sprout up again in the warmer months. “It’ll pick up again in the spring and summer. Always does.”

Police Commissioner Ray Kelly was beaming over the streak.

“I like it,” he said with a wide smile after the department’s promotions ceremony yesterday at Police Headquarters.

Kelly said cold weather and rain often impact the crime rate.

“That’s sort of the anecdotal answer we have in policing,” Kelly said about the current cold snap’s effect on the murder rate.

“There’s no single easy answer, but to the extent that weather played a role, we’re ready for more cold weather,” Kelly joked about the latest homicide freeze-out.

The only potential snag in the NYPD’s run is a woman found dead yesterday in Brooklyn. She was half-naked, and cops were calling her death “suspicious.”

But police said that there were no obvious signs of trauma and that the city medical examiner would determine the cause of death.

Other notable homicide-free streaks were in August 2010, when the city went seven days between murders. And in 2009, there was a six-day stretch with no murders, police said.

So far in 2013, New York City has logged only 18 murders — compared with 23 for the same period in 2012 — for a decrease of 21.7 percent, records show.

There were just 414 homicides in 2012 — an NYPD record that the department chalked up to operations that flood high-crime areas with patrol officers.